Most automotive air bag restraint systems, presently in use, use gas generant compositions in which sodium azide is the principal fuel. Because of disadvantages with sodium azide, particularly instability in the presence of metallic impurities and toxicity, which presents a disposal problem for unfired gas generators, there is a desire to develop non-azide gas generant systems, and a number of non-azide formulations have been proposed. However, to date, non-azide gas generants have not made significant commercial inroads.
Alternatives to azides which have been proposed, e.g., in U.S. Pat. No. 5,035,757, the teachings of which are incorporated herein by reference, include azole compounds, including tetrazole and triazole compounds. Tetrazole compounds include 5-amino tetrazole (AT), tetrazole, bitetrazole and metal salts of these compounds. Triazole compounds include 1,2,4-triazole-5-one, 3-nitro 1,2,4-triazole-5-one and metal salts of these compounds. Although all of the above azole compounds are useful fuels in accordance with the present invention, AT is the most commercially important of these.
Gas generant systems include, in addition to the fuel component, an oxidizer. Proposed oxidizers for use in conjunction with azole fuels include alkali and alkaline earth metal salts of nitrates, chlorates and perchlorates. A problem with azole compound-based gas generant systems, heretofore proposed, is their high combustion temperatures. Generated levels of toxic oxides, particularly CO and NO.sub.x depend upon the combustion temperature of the gas-generating reaction, higher levels of these toxic gases being produced at higher temperatures. Accordingly, it is desirable to produce gas generant mixtures which burn at lower temperatures.
Several gas generant processing procedures utilize water. Water-processing reduces hazards of processing gas generant materials. It is therefore desirable that gas generant compositions be formulated so as to facilitate water processing.
One Example of water processing, taught, e.g., in U.S. Pat. No. 5,015,309, the teachings of which are incorporated by reference, involves the steps of
1. Forming a slurry of the generant ingredients with water. PA1 2. Spray drying the slurry to form spherical prills of diameter 100-300 microns. PA1 3. Feeding the prills via gravity flow to a high speed rotary press. PA1 1. Forming a slurry of the generant ingredients with water. PA1 2. Extruding the slurry to form spaghetti like strands. PA1 3. Chopping and spheronizing the strands into prills. PA1 4. Tableting of the prills as described previously. PA1 The compound should be readily soluble in water, i.e., at least about 30 gm/100 ml. H.sub.2 O at 25.degree. C.; PA1 The compound should contain only elements selected from H, C, O and N; PA1 When formulated with an oxidizer to stoichiometrically yield carbon dioxide, nitrogen, and water, the gas yield should be greater than about 1.8 moles of gas per 100 grams of formulation; and PA1 When formulated with an oxidizer to stoichiometrically yield carbon dioxide, water and nitrogen, the theoretical chamber temperature at 1000 psi should be low, preferably, less than about 1800.degree. K.
In order to properly feed the tablet press, one needs well formed spherical prills. Without prills, plugging or bridging in the feed system is a common occurrence. Without prills, it is difficult to achieve uniform, high speed filling of the tablet press. These prills will not form in the spray drying step without at least a portion of the generant being water soluble. Typical slurries contain up to 35% water and it is preferred that at least 15% of the solid ingredients need to be soluble in the slurry.
Another common production technique, (e.g. U.S. Pat. No. 5,084,218), the teachings of which are incorporated herein by reference, involves the following steps:
The chopping and spheronizing step to form prills will not be successful unless a portion of the generant is water soluble.